The present invention relates to methods of catalytic partial oxidation (CPOX) of hydrocarbon fuels and, more particularly, to improved methods of CPOX of heavy hydrocarbon fuels having a substantial sulfur content, such as commercial and logistic fuels.
Interest continues in methods of using hydrocarbon fuels to produce a gaseous product stream of hydrogen and carbon monoxide, as well as using the gaseous product stream to fuel a fuel cell system, such as a solid oxide fuel cell system (SOFC). The studies concerning hydrocarbon processing vis-a-vis fuel cells have been numerous and include S. Ahmed et al., "Partial Oxidation Reformer Development For Fuel Cell Vehicles," Proceedings of the 32.sup.nd Intersociety Energy Conversion Engineering Conference, v.2, pp. 843-846 (1997); J. Bentley et al., "Reformer and Hydrogen Storage Development For Fuel Cell Vehicles," Annual Automotive Technology Development Contractor's Coordination Meeting (1995); British Gas plc, "Evaluation of the Potential Use of Partial Oxidation in Solid Polymer Fuel Cell Systems" (1997); L. Brown, "Survey of Processes for Producing Hydrogen Fuel from Different Sources for Automotive-Propulsion Fuel Cells," Los Alamos National Laboratory (1996); N. Edwards et al., "Fuel Cell System for Transport Applications including On-board HotSpot.TM. Reformer," Johnson Matthey Technology Centre, Blount's Court, Sonning Common, Reading RG4 9NH, United Kingdom; Elangovan et al., "Planar solid oxide fuel cell integrated system technology development," Journal of Power Sources, v. 71, pp.354-360 (1998); and T. Hirata et al., "Development of 60 kW Class Plate Reformer for Fuel Cell Plant," IHI Engineering Review, v. 29, no. 2, pp. 53-58 (1996).
The processes of converting hydrocarbon fuels to hydrocarbon/carbon monoxide gas products that have been developed in the past generally fall into one of three classes--steam reforming, partial oxidation (catalytic and non-catalytic), and auto-thermal reforming (a combination of steam reforming and partial oxidation). All three hydrocarbon conversion methods have been considered for use in conjunction with fuel cells, although L. Brown, supra, suggests that partial oxidation alone has not been favored. Nevertheless, the contemplated uses of fuel cells have been many, but significant attention has recently been given to transport vehicles. In that regard, fuel cells have been considered as replacements for internal combustion engines due to the advantages of greater efficiency and reduced emissions.
Despite their advantages, each of the three hydrocarbon conversion processes has design barriers. In the steam reforming method, which is endothermic, there are space and weight issues. Because steam reforming involves an endothermic reaction, an external source of heat is needed and the required heat transfer processes are slow. Of course, with the need for steam comes a concomitant need for a water supply or recycling. Any such additional items only add to the size and weight of a vehicle that can, in turn, affect other design considerations.
On the other hand, partial oxidation is an exothermic process and, therefore, does not have the disadvantage of requiring heat input and related transfer inefficiencies. There has been progress in the partial oxidation of light hydrocarbons (i.e., molecules with up to 5 carbon atoms) in recent years. But the technology for the conversion of complex or heavy hydrocarbon fuels (molecules with greater than 5 carbon atoms) to hydrogen and carbon monoxide is still in its early development.
Of great interest for fuel cells is the conversion of refinery liquid hydrocarbon fuels, such as gasoline and naphtha, to hydrogen/carbon monoxide gas streams by partial oxidation processes. Gasoline typically has a minimum of 80%-90% hydrocarbons with greater than five or more carbon atoms per molecule. For military applications, the hydrocarbon fuels of greatest interest are the so-called logistic fuels, such as JP-8 jet fuel, JP-4 jet fuel, JP-5 jet fuel and No. 2 fuel oil. In logistic fuels, the number of carbon atoms in a molecule may typically range from at least six and up to about 20 or more. But higher numbers of carbon atoms tend to increase the potential problem of carbon formation in the conversion process.
Carbon formation arises from the thermal cracking of hydrocarbons that can produce carbon-rich compounds (i.e., carbonaceous polymers) and, ultimately, coke. Thereby, system degradation can occur by, among other things, deposition of carbon on catalysts. In turn, the carbon deposition can lead to catalyst deactivation. Deposition on reactor walls can affect reactor performance and may lead to plugging. The problem of carbon formation has been extensively addressed in the past. Examples can be found in A. Dicks, "Hydrogen generation from natural gas for the fuel cell systems of tomorrow," Journal of Power Sources, v. 61, pp. 113-124 (1996); and W. Houghtby et al., "Development of the Adiabatic Reformer to Process No. 2 Fuel Oil and Coal Derived Liquid Fuels" (1981).
In addition to carbon formation, the processes for liquid hydrocarbon fuel conversion to hydrogen/carbon monoxide gas streams may be affected by the sulfur that is usually present in these fuels. In both light and heavy hydrocarbon fuels, but particularly the latter, sulfur is present in varying amounts. The specifications for sulfur content in logistic fuels such as Jet A, JP-8, JP-4, JP-5, etc. is given by military specification MIL-T-5634M/N. These specifications require the maximum amount of total sulfur content in the fuel to be 0.3 wt. % (tested according to standardized methods D1266/D1522/D2622). Typically, however, commercially available jet fuels have a total sulfur content of about 0.05-0.07 wt. %. The compounds of sulfur which remain in the liquid refinery streams are usually the refractory benzothiophene, dibenzothiophene, and their derivatives [Lee et al. "Removal of Sulfur Contaminants in Methanol for Fuel Cell Applications," Fuel Cell Seminar Poster Session, (1996)], which are essentially difficult to remove. As with carbon formation, sulfur can poison the catalyst and do so to a point where the catalyst becomes completely deactivated. Catalysts based upon nickel or platinum have appeared to be particularly susceptible to poisoning. It has been postulated that sulfur forms surface stable compounds with the catalyst. Thereby, catalyst active sites for oxidation are depleted and efficient production of hydrogen and carbon monoxide through catalytic partial oxidation is hindered.
One potential solution to the presence of sulfur has been to remove the sulfur prior to processing. Nickel or other transition metals, such as iron, have been known to remove sulfur from sulfur bearing organic compounds and are used in the laboratory. They have also been used as adsorbents to remove the thiophenic sulfur that remains in hydrocarbon fuels after hydro-desulfurization. These metals are very active for sulfur removal but suffer from the inability to adsorb a large quantity of sulfur because adsorption is limited only to the external surface of the metal due to the large size of the thiophenic molecule. Of these transition metals, Raney nickel seems to offer the best choice because of its high surface area. But experimental data shows that a ratio of about 100:1 by weight of Ni:S is needed for complete removal of the residual thiophenic sulfur from jet and diesel fuels. The required high Ni to S ratio limits this method of desulfurization to fuels with very small concentrations of sulfur, i.e., a few ppm of sulfur. For the removal of sulfur from logistic fuels that have hundreds of ppm of sulfur, desulfurization by nickel is costly and demanding in terms of metal weight and volume and, therefore, this method is impractical.
A solution around the sulfur problem in partial oxidation has been to omit the catalyst entirely (i.e., non-catalytic partial oxidation), particularly for converting heavy hydrocarbons on an industrial scale, and removing the sulfur, if necessary, after the partial oxidation reactor. But doing so trades process efficiencies, shorter processing times and milder temperature regimes provided by a catalyst for sulfur tolerance relative to the partial oxidation reactor. A general discussion of sulfur problems in reforming is found in A. D. Little, "Fuel Cell Powerplant System Considerations" (1996); and M. Stephanopoulos et al., "Conversion of Hydrocarbons for Fuel Cell Applications," JPL Publication 82-37 (1981).
In terms of further potential design problems associated with the catalyst, the catalyst has usually been a metal supported on some material. The supporting material has the potential for affecting heat transfer during and after the exothermic oxidation reaction. Providing a less than optimum heat transfer can lead to system performance degradation. For example, in pellet catalyst beds used in the past, hot spots and non-reactive regions can exist. With an open channel structure such as a honeycomb monolith, on the other hand, the connecting walls provide more uniform temperature than the pellet bed. Also, the monolith can respond faster to load changes due to its better heat transfer properties. Some of the monoliths have been constructed of metal or ceramic. Discussions of catalysts are found in U.S. Pat. Nos. 5,658,497; 5,639,401; 5,510,056; and 4,844,837.
Given the many potential design barriers to catalytic partial oxidation, it is not surprising that most of the research has been in the area of very light hydrocarbons (C.sub.1 to C.sub.5 hydrocarbons), which are easier to reform in comparison to heavy hydrocarbons. One example of light hydrocarbon processing by CPOX includes D. Goetsch et al., "Microsecond Catalytic Partial Oxidation of Alkanes," Science, v. 271, pp.1560-1562 (1996). Therein, a single layer, Pt-10% Rh gauze was used in a reactor with contact times as short as 10 microseconds to partially oxidize ethane, propane, butane and iso-butane to olefins and oxygenates. U.S. Pat. No. 5,654,491 is related to the above.
In another example of converting very light hydrocarbons by CPOX, L. Schmidt et al. "Partial oxidation of CH.sub.4 and C.sub.2 H.sub.6 over noble metal-coated monoliths," Catalysis Today, v.21, pp.443-454 (1994) describe the use of a noble metal film, such as rhodium, on a monolith catalyst of alpha-alumina foam to produce syngas, i.e., hydrogen/carbon monoxide mixtures. Metal loading was from about 1 to 20 wt. %. Contact time between the feed and catalyst ranged from about 10.sup.-4 to 10.sup.-2 seconds while operating at about 1000.degree. C. in pure oxygen gas. No steam was used in the process. With a methane fuel to oxygen ratio of to 2:1, Schmidt et al. reported 90% syngas production. In a related fashion is U.S. Pat. No. 5,648,582 wherein L. Schmidt et al. indicate that catalyst composition is the critical factor in CPOX, as opposed to mass transfer rate.
Additional examples of light hydrocarbon conversion by CPOX and other methods are found in U.S. Pat. Nos. 5,720,901; 5,486,313; 5,421,842; 4,789,384 and European Patent App. No. 0-0225-143.
In contrast to very light hydrocarbons, a heavier hydrocarbon fuel--JP-8 jet fuel--was partially oxidized to fuel an SOFC in S. Elangovan et al., supra. The JP-8 fuel was apparently considered to have a sulfur content of about 0.3 wt. %. The depicted system includes the use of steam, a soot filter downstream of the reactor and a desulfurizer downstream of the soot filter. The prediction was that the system would achieve a 75% cold gas efficiency, i.e., higher heating value of (H.sub.2 +CO) products/higher heating value of feedstock.
Another example of heavy hydrocarbon processing is shown in U.S. Pat. No. 4,087,259 wherein a rhodium catalyst on spherical gamma alumina catalyst carrier packed in a packed bed configuration was used to process gasoline and naphtha, although other hydrocarbons were mentioned as being suitable. No specifics of a sulfur content, if any, in the hydrocarbon were given. The rhodium was present at about 0.0005 to 1.0 wt. %, with a concentration in excess of 1.0 wt. % providing no increase in catalytic activity. A reaction temperature of about 690.degree. C. to 900.degree. C. was maintained. Above about 900.degree. C., thermal decomposition occurred and was stated to be a prohibited temperature regime. The liquid hourly space velocity (LHSV) for the partial oxidation was kept at about 0.5 to 25 h.sup.-1. A carbon to oxygen ratio was maintained at about 0.7 to 1.8.
Further examples of heavy hydrocarbon processing by non-catalytic partial oxidation, catalytic partial oxidation, gasification, and other methods are found in U.S. Pat. Nos. 4,778,484; 4,522,894; 4,244,811; and 4,115,074.
A catalytic partial oxidation process for the conversion of heavier hydrocarbon fuels, and especially logistic fuels, to hydrogen/carbon monoxide is needed which can operate in the substantial absence of steam, thereby simplifying the overall system design. In particular, there is a need for a method of processing heavy hydrocarbons having a number of carbons in excess of five. Additionally, there is a need for a heavy hydrocarbon fuel processing catalytic partial oxidation process that can provide a lightweight, compact, robust and durable source of hydrogen and carbon monoxide that could be used to fuel a solid oxide fuel cell system. A CPOX process is also needed which can overcome the tendency of carbon formation from heavy hydrocarbons. There is also a need for the catalytic partial oxidation of hydrocarbon fuels having a substantial sulfur content, i.e., sulfur in excess of about 50 ppm by weight, without substantial catalyst deactivation. At the same time, a catalytic partial oxidation process is needed which can maintain desired operating performance in the presence of sulfur. Additionally, there is a specific need for such a process that can take place in the substantial absence of desulfurization prior to the oxidation reaction. A further need is for a CPOX process which can reform heavy hydrocarbons with substantial sulfur over an extended period of time, while maintaining a desired steady-state yield efficiency.
As can be seen from the above discussions, there is a substantial need for an improved method of CPOX and a method of supplying a hydrogen/carbon monoxide fuel to a fuel cell system, such as a solid oxide fuel cell system.